The present application claims priority of Chinese patent application Serial No. 200610171613.7, filed Dec. 31, 2006, the content of which is hereby incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to terahertz time-domain spectroscopy analysis, and more particularly, to a method and an apparatus for assessing the purity of vegetable oils by mean of terahertz (hereinafter referred as “THz” for short) time-domain spectroscopy.
2. Description of the Prior Art
Edible vegetable oils are necessary to people, but the prices thereof vary greatly dependent on the categories and nutritive values. In recent years, some illegal producers incorporate some low-cost vegetable oils into high-cost vegetable oils to enlist profits, and thus some categories of adulterated vegetable oils appear in the market. It is a problem to be solved in the quality supervision that how to assess the truth of the vegetable oils and analyze the category of the adulterated vegetable oils and the adulterated amount. Therefore, an easy, quick and reliable assessment is needed in order to protect legal producers and customers.
The conventional method to assess vegetable oils is mostly dependent on the physical chemistry characteristics such as solidifying point, refractive index, degree of unsaturation and iodine value, but it is only roughly qualitative detection method. Currently, the technology that can make quantitative assessment on adulterated vegetable oils mainly includes chromatographic analysis and spectroscopic analysis methods.
Document 1 (WEI Ming, etc., A New Adulteration Detection Method on Edible Vegetable Oils by Gas Chromatography, Food Science, 2003, 24(12), 103-106) discloses a method for assessing the vegetable oil adulteration by measuring composition and content of fatty acid in the vegetable oil through gas chromatography. Gas chromatography is a physical separation method, and the principle thereof is that different substances have different distribution coefficients in a system composed of two phases, i.e., stationary phase and mobile phase. When these two phases move relative to each other, these substances move along with the mobile phase, and distribute between these two phases repeatedly. Therefore it is possible to make the substances whose distribution coefficients have tiny difference have greatly different moving speeds, so that respective components are completely separated. Vegetable oils mainly are some categories of glyceride made of fatty acid such as palmitic acid, stearic acid, oleic acid, linoleic acid and the likes, and composition and content of the fatty acid of different categories of vegetable oil are different, thus the composition and content of fatty acid will change after adulteration. The purpose of discriminating the category of adulterated oil and computing the adulterated amount can be achieved by assessing composition of the fatty acid by means of gas chromatography, and comparing with that of a category of pure oil.
However, when gas chromatography is used to assess composition and content of fatty acid, it is necessary to methyl-esterify the fatty acid first. Currently, a vitriol-methanol or potassium hydroxide-methanol system is used to perform methyl-esterification under normal temperature or warmed condition. However, this procedure takes some time (generally 20˜30 minutes), which does not meet the requirement of practical application for rapidly detecting batch of samples. Furthermore, this method is a destructive method, that is to say, this method changes the composition of the detected sample, and so the sample can not be reutilized.
Document 2 (E. C. López-Díez, et al. Rapid quantitative assessment of the adulteration of virgin olive oils with hazelnut oils using Raman spectroscopy and chemometrics. <Journal of Agricultural and Food Chemistry>, 2003, 51(21):6145-6150) discloses a rapid quantitative assessment of the adulteration of olive oils using Raman spectroscopy and chemometrics. Raman spectroscopy is a kind of molecular vibration spectroscopy, and the physical basis is Raman scattering effect induced by inelastic scattering of molecules to incident light. Position of Raman spectral line reflects the molecular structure characteristic and can be used in qualitative analysis because difference chemical bonds or radicals have different vibrations; intensity of Raman spectral line is in proportion to the intensity of incident light and concentration of sample molecules, and can be used as basis of qualitative analysis. Document 2 utilized a dispersive instrument employing a laser to measure Raman spectra of high quality virgin olive oils and refined hazelnut oils first, and discriminated these two vegetable oils whose chemical properties are very close by using the spectra data as eigenvectors. The model of the Raman spectra data of virgin olive oils adulterated with hazelnut oils in different concentrations was constructed through the Partial Least Squares algorithm, and the purity of the olive oils was predicted by the constructed model.
As stated above, Raman spectrum, which is considered as a fingerprint of a given oil, can reflect the structure characteristic of molecules, and can make rapid quantitative assessment of the adulteration of vegetable oils in combination with chemometrics. This method will not change the composition of the sample, and neither preliminary treatment of the sample nor complicated sample preparation procedure is needed. However, the disadvantage of Raman spectroscopy is in that the efficiency of Raman scattering is low and the scattered light is so weak as to be overwhelmed by the fluorescence generated by the sample or the impurity in the sample, whereby the detection accuracy will be affected.
THz technology is an advanced technology that is newly developed, which relates to generation, detection and application of THz radiation (commonly refers to electromagnetic waves with a frequency range of 0.1-10 THz. THz-TDS is one of the most important technologies of terahertz radiation applications, and the basic idea is to obtain absorption and dispersion spectrum of the sample in THz region by measuring the waveforms of THz pulses before and after transmitting the sample. Many organic molecules have strong absorption and dispersion in THz frequency range due to vibrational and rotational transition of dipoles. THz spectra of different substances often reveal specific features, and provide unique identification information fingerprint) for the molecule conformation, by means of which the substance components can be discriminated. Therefore, this technology can be widely used in various fields such as quality control and security inspection